224 SUMMARY. 



General review of the characters of the Molluscan larvce. 



The typical larva of a Mollusc, as has been more especially pointed 

 out by Lankester, is essentially similar to the larva of a number of 

 invertebrate types, and especially the Chffitopoda, with the addition 

 of certain special organs characteristic of the Mollusca. 



It has a bent alimentary tract, with a mouth on the ventral 

 surface and a terminal or ventral anus. The alimentary tract is 

 divided into three regions : oesophagus, stomach, and intestine. 

 There is a variously developed pra j or-al lobe with a ring of cilia the 

 velum, and a perianal lobe, often with a patch of cilia (Paludina, 

 etc.). In all these characters it is 'essentially similar to a Chcetopod 

 larva. The two characteristic molluscan organs are (1) a foot be- 

 tween the mouth and anus, and (2) an invagination of the epiblast 

 on the dorsal side at the hinder end of the body, which is connected 

 with the formation of the shell. 



The larvae of most Gasteropoda, Pteropoda, and Lamellibranchiata 

 present no features which call for special remark ; but the larva; of 

 the Gymnosomata amongst the Pteropoda, and of the Scaphopoda, 

 Polyplacophora and Cephalopoda present interesting peculiarities. 



The larvae of the Gynmosomata are peculiar in the presence of 

 three transverse ciliated rings, situated behind the velum (Fig. 109). 

 These rings might be regarded as indications of a rudimentary seg- 

 mentation; but, as already indicated, this view is not satisfactory. 

 There is every reason for thinking that these rings have been 

 specially acquired by these larvae. 



At first sight the larvae of the Gymnosomata might be supposed 

 to resemble those of the Scaphopoda, which are also provided with 

 transverse ciliated rings ; but, as shewn above, the rings of the 

 Scaphopoda are merely parts of the extended velar ring. 



Thus, the ciliated rings of the two larvae so similar in appear- 

 ance are in reality structures of entirely different values, being in 

 the one case parts of the velum, and in the other special develop- 

 ments of cilia behind the velum. 



The great peculiarity of the early larva of the Scaphopoda is the 

 enormous development of the praeoral lobe, which gives room for the 

 development of the ciliated rings. In the presence of a central tuft 

 of cilia, at the anterior extremity, the larva of the Scaphopoda 

 resembles that of the Lamellibranchiata, etc. 



The larva of the Polyplacophora resembles that of Lamellibranchiata 

 in its anterior flagellum, and that of the Scaphopoda in the large de- 

 velopment of the praeoral lobe ; but is quite peculiar amongst Mollusca 

 in the transverse segmentation of the mantle area. 



The embryo of the Cephalopoda agrees very closely with that of 

 normal Odontophora in the formation of the mantle and (?) of the 

 shell-gland, but is quite exceptional (1) in the almost invariable 

 presence of a more or less developed external yolk -sack, (2) in the 



